Robert Koch,1 in his first article on tuberculin, noted the diagnostic possibilities of this substance. He was, however, chiefly interested in its therapeutic aspects, and although he described both the local and the constitutional reactions to injections, the widespread and harmless application of tuberculin as a diagnostic agent was not appreciated until Pirquet,2 in 1907, brought forth the now familiar skin test.

Koch originally prepared tuberculin by concentrating an old glycerin broth culture of tubercle bacilli over steam and filtering it through a porcelain filter. This preparation has always been designated "Koch's old tuberculin" (O. T.).

Of the many modifications of old tuberculin that have been proposed, none appear to have any striking points of superiority, and therefore the original product is still almost uniformly the one of choice. Two recent reports, however, should be mentioned. The first is concerning a protein derived from tubercle bacilli. This was